Abstract : Investigated are the behaviors of the carbonaceous mesophase that significantly influence the processing of carbon-carbon composites. An understanding of those behaviors will provide insights for more effective and economical fabrication of such composites. This first annual report summarizes micrographic findings obtained during the development of experimental procedures for observing how mesophase morphologies are formed within carbon fiber bundles at various stages in pyrolysis. Studies of room-pressure pyrolysis identified the following patterns of mesophase formation within fiber bundles. The mesophase transformation proceeds more slowly within a bundle, and without the extensive deformation that characterizes mesophase pyrolyzed in bulk. In the early stages of transformation, the fiber is wetted by both mesophase and pitch, a behavior that appears to be independent of fiber type, at least for the four fibers tested (PAN-based fiber: T300; mesophase-based fiber: VSA-11, P55, and P100). The coarseness of the porosity developed within a bundle seems to be determined by the extent to which the fiber bundle is restrained. Individual filaments in open, unconstrained bundles can apparently move locally in response to interfacial tensions, to produce regions in which the filaments are more closely packed.